atmosphere of Venus

South polar region of Venus. Image in UV by ESA's
Venus Express probe.

Artist's impression of the surface of Venus

Venus has a dense atmosphere, composed chiefly
of carbon dioxide, which generates
a surface pressure 90 times greater than that on Earth. This massive blanket
of carbon dioxide is also responsible for a runaway
greenhouse effect that heats the planet's surface to an average temperature
of 467°C (872°F) – hot enough to melt lead.

Venus' atmosphere consists almost entirely (96.5%) of carbon dioxide, with
clouds containing droplets of sulfuric
acid along with compounds of chlorine
and fluorine. These precipitate an acid
rain called virga, which evaporates before it has the chance to reach the
surface. In the upper part of the atmosphere, clouds swirl by at a rate
of 300 km/h, driven by fierce winds.

History of observations

That Venus was permanently enveloped in what appeared to be thick, white
clouds soon became clear following the development of the telescope. By
analogy with the Earth, it was generally assumed that these clouds were
made of water vapor, a conclusion supported by early spectroscopic
studies by Secchi. The main composition of
the atmosphere remained a matter for conjecture until spectroscopic observations
by Walter Adams and Theodore Dunham,
in 1932, established that it was carbon
dioxide. Speculation about the make-up of the clouds continued. In 1937,
R. Wildt suggested methanal (formaldehyde),
while in 1954, Fred Whipple and Donald Menzel
proposed ice crystals. Today, they are known to consist primarily of sulfuric
acid.

Hydroxyl radical in Venusian atmosphere

In the May 2008 issue of Astronomy & Astrophysics, Picconi et al.
report the first detection of the hydroxyl (-OH) radical in the night-side
airglow emission of Venus. The discovery was made using an instrument onboard
Venus Express. This is the first detection of -OH in the atmosphere of a
planet other than Earth. It is important because it will give new insight
into the dynamics of the upper atmosphere and the climate system of Venus.

Cold layer

Although the surface of Venus swelters at an average temperature of 467°C,
and much of the planet's atmosphere is also hot, there is a layer within
the atmosphere that is surprisingly cold – colder, in fact, than anywhere
in the Earth's atmosphere.[1] At an altitude of about 125 kilometers
above the surface, measurements by the Venus Express probe have shown, the
temperature drops to an amazingly low -175°C, cool enough in theory
for carbon dioxide ice or snow to form.